1. Field of the Invention
The present invention relates to 2-amino-5-nitropyridinium salts usable in non-linear optics and in electrooptics.
2. Description of the Background
By the term "nonlinear optics," is meant the field of optics which extends from the conversion of optical frequencies, i.e., obtaining an optical radiation from two radiations of different frequencies, the frequency of the conversion radiation being equal to the sum or to the difference of the frequencies of the two radiations or conversely, breakdown of an incident beam into two beams of low frequencies, to electrooptical modulation, i.e., modification of one of the radiation characteristics by application of an electric field to a crystal through which the radiation passes.
Many materials likely to be suitable for the conversion of optical frequencies or for the electrooptical modulation are already known. Of the latter, mineral materials such as potassium diphosphate (KDP), lithium niobate and potassium titanium oxide monophosphate (KTP) may be mentioned.
However, these mineral materials are, in fact, not effective, which necessitates using them in large thicknesses. For several years, research has been conducted relating to the production of crystals of organic molecules exhibiting an improved effectiveness relative to the inorganic crystals of potassium diphosphate or lithium niobate. Thus, as described by J. Zyss in Current Trends in Optics, Taylor & Francis, London, 1981, pp. 122-134, it was found that the aniline or pyridine-N-oxide derivatives such as metanitroaniline (mNA), 2-methyl-4-nitroaniline (MNA), methyl 2(2,4-dinitrophenyl)-aminopropanoate and 3-methyl-4-nitropyridine-1-oxide (POM) that are the object of FR-A-2 472 201 may be used for this purpose.
In EP-A- 0 091 838, the advantageous properties of the optionally deuterated N-(4-nitrophenyl)-L-prolinol (NPP) are also described.
Recently, in Zeitschrift fuer Kristallogr. 190, 1990, pp. 19-32, R. Masse and A. Durif proposed new piezoelectric and ferroelectric crystals, usable in nonlinear optics, consisting of methylalaninium diacid monophosphates. These monophosphates are polar materials with a mineral anionic framework, using polyanion (H.sub.2 PO.sub.4.sup.-).sub.n in the form of layers inside which organic cations are placed.
The search for salts for quadratic nonlinear optics, in which cations exhibiting a nature of intramolecular charge transfer and coupling are associated with chiral, achiral or mineral organic anions, has also been developed by G. R. Meredith (ACS symposium series 233, 1983, pp. 27-56). In this case, the object is to, at once, obtain:
1) a noncentrosymmetric stack of achiral organic cations in a crystalline structure, and PA1 2) an optimal orientation of these cations (charge transfer axis) relative to the dielectric axes of the crystal, making it possible to assure the birefringence necessary for the agreement of phases between the interacting beams, for each class of symmetry according to J. Zyss and J. L. Oudar (Pys. Rev. A 26, 2028, 1982). PA1 1) the nonlinear effectiveness, i.e., parametric or electrooptic figure of merit, PA1 2) the extent of the transparency range, PA1 3) the optical damage threshold, i.e., under continuous radiation or radiation by pulse, PA1 4) the mechanical qualities, i.e., hardness, strength, tendency to cleavage, density of dislocations, capacity for cutting and polishing, PA1 5) the chemical or physicochemical stability, i.e., photoreactivity, stability in air, hygroscopy, tendency to sublimation, and PA1 6) the capacity for crystal growth, i.e., solubility, stability in melting, initial purity.
Thus, two large families of materials have been the object of intensive research conducted independently until now, i.e., the mineral monocrystals, on the one hand, and the organic molecular crystals, on the other hand.
Each of these families exhibits advantages and drawbacks connected to such parameters as:
Unfortunately, with respect to these various parameters, neither of the two families is completely satisfactory. Some crystalline organic materials are known for their figures of merit (parametric gain or half-wave voltage) that are clearly higher than those of their mineral equivalents. Thus, N-(4-nitrophenyl)-L-prolinol (NPP) has a parametric gain that is greater by two orders of magnitude than that of potassium titanium oxide monophosphate (KTP). However, the mineral oxides such as KTP appear to resist better the laser irradiation in continuous flux.
The organic materials studied to date in general appear to exhibit an optimum of the effectiveness-transparency compromise on the whole centered in the near infrared. However, the cohesion of the ionic mineral cages is greater than that of the molecular crystals associated by less energetic Van Der Waals forces. It is therefore possible to expect better mechanical properties. Moreover, the increase in aqueous solution of bulky mineral crystals, for example, potassium diacid monophosphate (KDP) is now a well-established industrial reality.
Clearly, a need exists for organic cation salts exhibiting the advantages of two types of materials, either, on the one hand, the high polarizabilities of organic molecules and the possibility, by virtue of chemical substitutions, of inducing high nonlinear capacities and, on the other hand, the cohesion and the strength of the stacks of the anionic cages of mineral oxides. To date, however, such organic cation salts are unknown.